1. Field of the Invention
The present invention relates to an image forming apparatus in which an image is formed on a recording material in accordance with an electrophotographic technology.
2. Description of the Related Art
In electrophotographic image forming apparatuses such as copiers and laser beam printers, an electrostatic image (latent image) is formed through irradiation of light, corresponding to image data, onto an electrophotographic photoconductor (photoconductor drum) that is charged homogeneously by a charging means. Toner (developer) is supplied from a developing assembly to the electrostatic image, and the electrostatic image is made visible in the form of a toner image. The toner image is transferred from the photoconductor to a recording material such as recording paper, by a transfer device. The toner image is then fixed on the recording material in the fixing apparatus, to form a recorded image on the recording material.
Known developing assemblies that are provided in such image forming apparatuses include developing assemblies that rely on a dry one-component developing method. In such a developing assembly, firstly a one-component developer (hereafter, toner) is supported on a developing roller (developer carrier), a uniform toner layer is formed by a layer-thickness regulating member, and the developing roller is brought into contact with the photoconductor. For instance, developing bias voltage formed of a DC component is then applied to the developing roller, to generate a potential difference between the electrostatic image on the photoconductor and the developing roller, and to develop the electrostatic image by causing the toner to move onto the electrostatic image (contact developing scheme).
In image forming apparatuses relying on contact developing schemes, technologies are known in which a potential difference (back contrast) between a photoconductor charging potential and a developing roller applied voltage, and a potential difference (developing contrast) between the developing roller applied voltage and a latent image potential, are appropriately set, to suppress fogging or the like (Japanese Patent Application Publication No. H08-171260).
In a case where the size of the developing device is to be reduced to cope with a reduction in apparatus size, the diameter of the developing roller has to be as small as possible. The developing roller that is pressed against the photoconductor drum deflects readily when the diameter of the developing roller is small. In a configuration where there is provided a toner supply roller for supplying toner to the developing roller, deflection of the developing roller can be suppressed by having the latter supported by the toner supply roller. However, a configuration in which support by a toner supply roller is lacking, through scrapping of the toner supply roller in order to reduce the size and price of the apparatus, entails significantly more pronounced deflection of the developing roller, which is a concern.
Ordinarily, at least the outermost layer of the developing roller is made up of an elastic rubber material, so as not to damage the photoconductor drum. When the developing roller flexes, however, the pressing force is stronger at the end sections than in the central section. As a result, the nip with respect to the photoconductor drum is thicker at the end sections, where the pressing force is strong, than at the central section (the width of the nip section in the recording material conveyance direction is larger at the end sections than at the central section).
In a case where the nip of the developing section was thick, reverse fogging might worsen, since the charge of the toner varied within the nip. Accordingly, the amount of fogging with respect to back contrast might be different between the central section and the end sections, in a case, as described above, where deflection was large and the nip width of the developing roller differed between the central section and the end sections.
FIG. 4 illustrates such a situation. FIG. 4 is a diagram illustrating a relationship between back contrast and fogging amount in a case where the nip width between the developing roller and photoconductor drum differs between the central section and the end sections. As FIG. 4 illustrates, reverse fogging worsens at end sections of large nip width. The value of back contrast for which fogging is minimal differs between the central section and the end sections, due to the influence of worsened reverse fogging. A possibility arose herein in that fogging might occur at paper portions at which the back contrast at the central section and the end sections cannot be optimized due to worsened fogging accompanying toner degradation, in the case of repeated printing over long periods of time using such a developing assembly.
Therefore, an image forming apparatus is demanded that allows suppressing occurrence of fogging yet more effectively.